Leibniz, as well as a number of other religious people, think that this world is the best possible world. And seriously holding that to be the case raises some problems. But first let me say a bit as to why anyone would think that this world is the best possible world. As they see it, it follows directly from believing that god is good and omnipotent. Now the first consequence of god being omnipotent and good is that he must be omniscient as well. Because if he wasn’t omniscient he would have to necessarily limit his power to areas in which he could predict its consequences (to do otherwise would be irresponsible. and hence not good), and thus would fail to be omnipotent.
Now, given that god is omnipotent, omniscient, and good he must create only the best possible world. Obviously god, from his act of creation, can see everything that will follow from it (perfectly predict the future). And because he is all powerful he can create the universe however he likes (or change it to be however he likes, if god comes into existence simultaneously with the universe). Thus everything that will ever happen is completely under god’s control. And if he is good that means that he will engineer things so that they turn out the best they possibly can (to allow things to be worse than they could otherwise be is an ethical failing). And thus this world is the best of all possible worlds.
Now the traditional way (really the only way) to try to escape this conclusion is to invoke free will, and claim that the future course of events isn’t completely under god’s control. Let us ignore for the moment that this contradicts god’s omniscience, since he would be unable to predict the future, and consider the possibility seriously. Here we can reason best by analogy. Assume I am creating a race of robots. Is it better for me to give them free will or program them deterministically? It seems clear that it is better to avoid giving them free will. You see if you give them free will they may very well choose to do evil. and you, as their creator would. be held at least partly accountable for that evil. Specifically if the robots do evil because they have free will someone could justly complain that we should have not given them this free will, and should have instead programmed them to be good. The only possible defense to this complaint is to insist that free will has some intrinsic value. And the only possible candidate for this value would be to claim that the robots couldn’t really be good without free will. But that is not a valid argument. Without going into a discussion of determinism, which reveals that people can be good and morally responsible for their actions even if those actions are completely determined, simply consider a robot programmed to want to be good, which has no ability to change that desire. Although this robot doesn’t really have free will, because it can’t want to be evil, it certainly is a good robot, by any standard, because of its deep desire to do good things. Thus we can conclude by analogy that if god is good he wouldn’t give us free will of the sort that lets our actions be unpredictable, and thus possibly lead to events turning out worse than the otherwise could have.
Thus we can conclude that if god exists, is good, and is omnipotent that this world is the best possible world, which as a corollary entails that every event that ever will happen has been determined since the beginning of time. And this raises a serious ethical problem. The deterministic nature of this world is not the problem that I am alluding to of course, although it is normally what is challenged. What I see as a problem is the fact that no matter what we do the consequences of our actions must have the effect, at least in the long run, of making the world a better place, or at least, in the long run, that they were the best possible actions we could have taken. This is because the world is the best possible world, so it is a contradiction to suppose that some other action would have made the world a better place. If some other action could have been better than we would have taken that action, because god gives us a guarantee, so to speak, that this world is the best possible world, and thus that all the best actions, from the big perspective, are taken. But that fact completely destroys ethical responsibility. No matter what action I take it is the best possible action I could have taken, no matter how reprehensible it may seem from our point of view. How then could we hold anyone accountable for their actions? Should we wish instead that they had acted in a way that would have made the world worse? But that is contrary to ethics, which motivates us to try and make the world a better place. Even if we judge them by their intent, someone, who has reasoned as above, may simply act on impulse, doing all sorts of horrible things, and reasoning that no matter what they do it must be for the best, otherwise god would have made the world so that they chose otherwise. Thus they may have intentions that we cannot fault, despite their horrible acts.
So god’s existing, being omnipotent, and being good completely eliminates ethics. This I take to be a reductio ad absurdum of the concept of god. If we wish to keep ethics, in any form, we must get rid of either god’s existence, his omnipotence or his goodness. Now clearly getting rid of god’s omnipotence is absurd, because that is what differentiates god from arbitrary objects within the universe. And getting rid of his goodness is also absurd, because it undercuts the motivation to believe in and worship god in the first place. And so we are left with the conclusion that if we are to save ethics that we must do away with god (accept that he doesn’t exist). Since the existence of ethics is more certain than the existence of god we have reason to believe that god does not exist. QED